Roots blower



SePt- .17 1935 G. E. A. HALLETT 2,014,932

- RooTsBLowEn Filed March 17. 1953 2 sheets-sheet 2 'W d", www@ Patented Sept. 17, 1935 ROOTS BLOWER George E. A. Hallett, Detroit, Mich., assigner, by

mesne assignments, to General Motors Corporation, Detroit, Mich., a corporation of Dela- Ware Application March 17, 1933, Serial No. 661,338 l 4 Claims.

This invention relates to a blower or supercharger of the well known Roots type having interm'eshing teeth or lobes which take in a volume of fluid between the tooth spaces and carry it 5 round between the teeth and a closely encircling housing, to be squeezed out as the teeth come into meshing relationship on the delivery side. Such blowers can be made to combine to a degree, the advantages of positive displacement and capacity for high speed operation without lubrication of the parts swept by the air or other uid which is being delivered by theblower. In order to eliminate rubbing friction, therotors of blowers of this type have been so dimensioned as to provide a small clearance between themselves and between themselves and their housing and have been driven in proper registry with one another by gearing so that the rotor lobes or teeth are relieved vof any driving duty and thesmall clearance of a few thousandths of an inch insures that the surfaces of the rotorsrthemselves do not touch either each other or their housingat any point in their operation. Although hereinafter the rotors will be referred to as forming a seal between themselves and their housing against the ow of iiuid between each other or between themselves and their housing, it will be understood that there is a small clearance and that the rotors and the housing do not actually touch each other atlany point in their operation.

As ordinarily designed, these blowers are quite suitable for low outlet pressures at constant speed and where a continuous discharge characteristic is not essential. a

Generally-Vit is the object of the invention to make a Roots type-blowerv more suited to the reh lquirements for scavenging," charging or supercharging an internal combustion engine.

The above and other objectsv are attained inthe manner hereafter described.

It has been found that for a reasonably large and unrestricted port area'together with the largest displacement volumel for a blower of given dimensions a three lobe rotor is best. That is, it has been found that a continuous discharge char- (Cl. 23o-141) acteristic together vwith maximum volumetric displacement, and with provision for sufficiently `large and unrestricted inlet and outlet port areas to give high volumetric eiciency, isbest attained in a blower of this type with a three-lobed rotor. 5'.

With straight toothed rotors, each having three lobes at 120 asabove, there will be cyclic discharge uctuations or pulsations every of rotation of the rotors, or six pulsations per revolution of the blower. The magnitude of the discharge fluctuations or pulsations is reduced as the number of lobes and the number ofl pulsa- 15 tions is increased but the displacement per unit of space occupied is also reduced. A three lobe rotor best eiects a compromise between the requirementsof maximum displacement for a blower of given dimensions and a maximum frequency of pulsations of lesser magnitude.

The volumetric displacement will vary with the lobe or tooth contour and while a cycloidal tooth form would give .the maximum volumetric displacement together with a seal preventing 25 any flow between the rotors themselves and their housing, itcannot do so except by taking away necessary stock at the boss required for strength. Since, however, the rotors are geared together independently of themselves and a cycloidal lobe o. contour for driving is no longera requirement, it is sufficient that there should at all times be a seal preventing any iiow of fluid between the rotors, and since this sealing can be eiected by a following edge when the leading edge of the tooth has ceased to make a seal there isa considerable part of the tooth contour performing no useful function. Advantage' is taken of this fact. to'modify 'the tooth, profile so that' adequate 4o strength at the bossis provided together with.

. only a minor reduction of the swept volume. It

has Vbeen found thata tooth tip having a contour which is a true arc of a circlelbest eects the required compromise. In order -to reduce or eliminate the noises of pneumatic origin which are due. to the pulsations or disturbances in the air stream inherent in the usual form of Roots type blower with straight' lobes Aor teeth and port edges parallel thereto giving sharp cut off and sharp opening, some means of approaching a more continuous and constant discharge isf required.

According to the vinvention this is attained by making the rotors of spiral or helical form around their own axes throughout their length.

` In this way the discharge variations, which with three lobed rotors occur in cycles of 60 of rotor revolution, will be smoothed out and the helical angle given to the rotors will eliminate both sharp cut off and sharp opening of the entire rotor length and the sudden back iiow into the low pressure chambers which goes therewith and at the same time reduce the extent or degree of fluctuations in the cyclic pulsations.

It has been found that these results are best attained when the helical angle is such that the projection of one end of the rotor in a plane at right angles to the axis of the rotr is displaced angularly relatively to the other end of the rotor a number of degrees equal to half the angle between adjacent teeth on the rotor, i.'e. the helical angle should be a function of the number of teeth on the rotors and equal to 360 divided by twice the number of teeth on each rotor. With three lobes at 120 on each rotor this helical angle will be 60 and a seal of 180 around the housing will be required for each rotor.

One dimension of the port areas must be divided equally above and below the transverse geometrical center line of the blower for equal displacement from each rotor and for maximum port areas with a seal of 180 this dimension will be equal to the distance between rotor centers for each port if the housing seals for each rotor are divided symmetrically 90 on either side It win .thus be seen that a three iebed rotor with one end angularly displaced 60 relatively to vthe otherbest fulfills the conflicting requirements for a blower of given dimensions, of maximum displacement, maximum number of pulsations of minimum magnitude per revolution.

and continuity of discharge'with gradual intake, exhaust, back flow and cut ofi, contributing to a smoother and more constantoutput..

A blower built in accordance with the invention and capable of operating at speeds up to 9.000 R. P. M. with a capacity of 900 cubic fed per minute and pressures up to 14 or 1'1 lbs. per

square inch above atmospheric pressure may have a temperature difference between inlet and outlet which are only 4" or 5 apart, inV theneighborhcod of 300 F. The intake air, particularly if mixed with gasoline, may be 15 F. below atmospheric temperature while the heat of compression may raise this temperature to 350? F.- at the outlet. The housing is water jacketed for the purpose of reducing this temperature difference, but it is not practicable so to cool the rotors. For this reason it has been found expedient to make the rotors and the housing of materials of di'fierentcoeilicient of expansion such that .although the housing is cooled it will expand approximately the same amountas the uncool'edrotors. For instance, cast iron may be used for the 'rotors and aluminum for the housing.

'I'he water jacketing 'of the housing is insufficient to equalize th'e temperature differenceV between inlet and outlet to keep thev aluminum der always to maintain the necessary clearance housing from unequal expansion out of shape. The hotter outlet side expands, giving more clearance between itself and the rotors and the inlet side buckles in towards the rotors. In orbetween rotors and housing within the desirable maximum andminimum limitations, and in order that these clearances may be approximately equal and as small as possible, particularly on the outlet or pressure side to reduce back iiov/ l0 between the rotors and their housing for the average operating conditions of speed and pressure and hence of temperature difference, the housing is preferably bored eccentric to the line of centers of the rotor shafts, so as to allow more clearance on the intake side when the housing has an even temperature throughout and there may be a slight excess of clearance on the outlet side when the temperature difference is a maximum. 20

In order to reduce mechanical noise, the rotors are synchronously driven through helical gears at each end of the rotors. 'I'he helical angle of the gear at one end of each rotor is opposed to that at the other end in order to balance end 25 thrust. 'I'he use of such gears at each end of the rotors prevents torsional displacement or twist of one end of each rotor relative to the other end and it maintains the rotor ends in registry independently of the rotors themselves. 30

The accompanying drawings show a Roots type blower constructed in accordance with the invention.

In the drawings.:

Figure 1 is a part cut away, part sectional ele- 35 vation of the blower.

Figure 2 is an enlarged end elevation showing the mounting of the driving gears.

Figure 3 is a section on line 3-3 of Figure 2.

Figure 4 is a section on line 4--4 of Figure 1. 40

Figure 5 is an enlarged part detail sectional view of the sealing rings between the rotating spindles of the rotors and the housing to prevent the escape of fluid out from the blower and the leakage of lubricating oil for the gears and 45 the bearings into the blower. The rotors I and 2 each with three lobes, or teeth, 3, l, 5 and 6, 1, 8 of right and left hand spiral form respectively, the projection of one end of -each rotor in a plane at right angles to the axis of the rotor being displaced angularly relatively to the other end, are provided 'with splined stub shafts such as 9, Ill, II, splined in the rotor hubs and pinned against endwise movement4 relative to the rotors as by a pin I2. 55

,The rotor housing I3 closely enclrcles 180 of each rotor and follows the two tangents common to both rotor tip circles. It is provided with intake and output ducts 'Il and I5 leading to inlet and outlet ports I6 and I 1, extending a distance 60 equal in length to the distance between centers of the rotors, divided equally above and below the transverse geometrical center line of the blower along each of the tangents common to the two rotor tip circles. A water cooling space I9 around the blower is provided. End plates 20 and 2I form theend closures for the rotor housing Il'.

Splined on the rotor shafts are gear hubs 22,

23, 24, and 25 for the right and left hand spiral 70 gears 26 kand 21 of the rotor I and the left and right hand spiral gears 2l and 29 of the rotor 2. On the rotor shafts between the rotor hubs and gear hubs are end thrust collars 3l forming part of an ou seal later to be described. 'rne end 5 I are clamped up tightly against the rotor hubs by thrust collars 30 and the gear hubs 22, 24, and 25 retaining rings 3| and studs 32 screwed into the endsof the rotor shafts. A spiral gear 33 with its hub 34 and stud 35 forms a similar clamping means for the gear hub 23 and its .end thrust collar 30. The gear 33 forms a convenient drive for a gear 34 which, inthe example illustrated, drives through the medium of the cam 36 a gasoline pump not shown.

The splined' hub 24 for the gear 28 of the rotor 2 is elongated andadapted' to take the drive for `the blower through the medium of a splinedy quillor driving shaft, not shown.

Bearings 31 held in the end plates 20 and 2I by studs 38 carry the gear hubs and hence the rotors.

'I'he spiralgears 26, 21, 28, and 29,- are each dowelled to their hubs, with the rotors in proper phase relationship by dowels 39 retained in position by a plate 40 secured by two of four cap screws 4.1 which hold each gear rmly on its hub.

An end cover plate 42 encloses the gears 21 and 29 and is provided with an air vent 43. No such end cover plate for the gears 26 and 28 is shown as in' the present example this end of the 'blower bolts up to the crankcase of an internal combustion engine with the blower gear chamber in communication with the engine crank A lubricating oil passage 44 through the end plate housing I3 and end plate 2| .supplies oil `to the bearings and the gears Awhile the gear chambers at each end of the rotors are oo nnected. together by an o il passage 45.

In order to seal the,y rotor bearings against leakage ofiluid out from the blower andsagainst leakage of oil from the bearings into the blower, each bearing 31 is provided with a seal between the rotating end thrust collars of the rotors and the stationary end plate, asvshown more clearly in Figure 5. Referring to this figure, the

thrust collar 30 has a carbon insert ring 415.

ring- 49. The seal is thus formed between the stationary inner steel ring and the rotating ca'ry bon insert, ring.

This arrangement has been found adequate and is in a: measure self lubricat-` ing. Since the pressurein any part of the blower `is always atmospheric or over,any leakage out-.-

ward will press the sealing .surfaces 4I` and 49 more tightly together by pressure on the dia- Dhlagm 4l.

An oil bleed yso'i's armed in each bearing 31 in,

order to' lead any oil` which leaks between the t bearing 31 and the thrust collar back to the gear chamber. I

The lobes of the rotors are cored out hollow asl shown in Figure 4, and before assembly are.

balanced by removing material as required frdm' the inside of the lobes,l suiiicient stock being provided for this purpose. Since the' blower may be to handle not only air, but fluid vapor, or gasoline, :for instance, which may condense in the hollow rotors and put. them out of balance,

' openings on opposite sides of the meshing zone of Ia. small. clearance and an effective'seal is at' all rial so as to provide for approximately similar small drain holes 5l are drilled in each lobe as indicated. 4

' 52 is a drain plug for lthe blower water jacket.

` I claim:

1. A Roots type blower having means adapted 5 to -provide a continuous discharge characteristic together with maximum volumetric displacement and with lprovision for ksufllciently large andmunrestricted inlet and outlet port areas to give high volumetric efliciency, said means comprising iii-'10 termeshing three-lobed rotors of helical form such that one end of each rotor is displaced angularly relative `to the other end, by an amount substantially equal to half the angle between adjacent lobes thereon, a housing for the rotors cooperating with each rotor 'so as to provide only a small clearance and an effective seal between itself and each rotor for an angle of the sweep of each rotor equal substantially to one and one-haii times the angle between adjacent lobes on each rotor and not more than 180, whereby a total area equal to at least the distance between centers of the rotors multiplied by the length of the rotors is available in the housing for each of the inlet and outlet port the gears, and gearing for driving and maintaining the rotors in registry independently of themselves -such that in. the operation of the blower times maintained between the rotors.

' 2. A Rootslblower according' to claim 1 and having opposed helical gears at each end of the rotors, whereby the rotors are driven and maintained in registry independently of themselves by preventing torsional displacement between the ends. of each rotor and whereby end thrust of the helical gears is neutralized and lend thrust n o'f the rotors is counteracted.

3. A blower according to claim l and having means for cooling the housing and for reducing the temperature differential between the inlet and outlet of the blower, the material of said housing having a coefllcient of expansion suificiently higher than ythat of the rotor mateaverage expansion of the rotors and cooled housing under operating conditions.

4. In a positive displacement type of blower,

vin combination, two intermeshin'g-three-lobed rotors of helical form, the projection of one end 5 of each rotor in a plane at right angles to the axis of the rotor being angularly displaced relatively to the other, a housing surrounding' the rotors andvforming circumferential sealing portions of 180 around each rotor, the housing;5

being. divided symmetrically about thevlines, of center of the rotor axes on either side thereof in aplane transverse to the rotor axes and laving ports in the housing extending a distance subw` stantially equal in length to the distance between centers of the roto'rs, divided equally above and below tbe-transverse1 geometrical 'center lineqof i the blower along each'f the tangents common to the circumferential sealing portions, lwhereby w I -a' continuous discharge charaiteristic together with maximum volumetric displacement and sub' nciently large and unrestricted inlet and outlet port vareas to give high volumetric -emciency is obtained while providing amegective seal bei tween the rotors and housing.

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